Wood and wood-base materials as well as other building materials, components and materials have to meet extensive requirements with regard to flameproof properties. Thus, for example, building materials can be classified according to DIN 4102, components for electrical equipment according to UL 94 or IEC-60695-2, components for railway vehicles according to DIN 5510 and furniture according to BS 5852 and can be made correspondingly flame-retardant for their use. Particular requirements are set, for example, for wood-base materials for the equipping of aircraft (e.g. FAR 25.853) or ships (e.g. IMO A.652(16)). An overview of numerous test and requirements is given, for example by Jürgen Troitzsch, “Plastics Flammability Handbook”, 2004, Carl Hanser Verlag, Munich.
In addition, the fire protection requirements are constantly increasing. Thus, for example, new European standards which are intended to replace the national test standards to date set substantially higher fire protection requirements. The SBI test (EN 13823) makes it necessary, for example, to take account of not only the fire behaviour but also the fume density. The new standard intended for railway vehicles (prEN 45545) takes into account, for example, fume density and fume toxicity. Requirements with regard to the fume toxicity, which is often determined via the measurement of, inter alia, the hydrogen halide concentrations in the fumes, can, for example, make it impossible to use tried and tested halogen-containing flameproofing compositions. In addition, halogen-free flameproofing compositions are also preferred because the public has more faith in them with regard to product safety and environmental protection. For many fields of use for wood-base materials, this means that the tried and tested and functioning flameproofing treatment must be revised with regard to new requirements.
The flameproofing treatment of wood-base materials by treatment with chemicals is known. The most important treatment methods are the pressure process and coating. Flame-retardant coating of wood has the considerable disadvantage that the natural material wood loses its typical natural appearance in the building sector or as furniture material. Even if the coating is transparent, the appearance of the wood surface and possibilities for further processing are adversely affected, which may be undesired. More frequently used, therefore, is the pressure process in which the wood is brought into contact with a generally aqueous solution of a flameproofing composition, the take-up of a sufficient amount of flameproofing composition into the volume of the wood is achieved by suitable measures and finally drying is effected. Predominantly used flameproofing compositions are phosphorus-, nitrogen- and boron-containing substances. Examples of these are ammonium phosphate, urea or sodium tetraborate.
The flame-retardant treatment of wood by the pressure process with its salt-like flameproofing compositions is technically established and economical but has a number of disadvantages. In the case of the direct action of water, for example on the outer skin of buildings, the flameproofing compositions are dissolved out and the flame retardance declines. For this reason, flameproofing compositions which are fixed in the wood and cannot be leached out by water are used in particular for exterior applications. Examples of this are combinations of flameproofing compositions with a resin which is completely polymerized in the wood after the impregnation and binds the flameproofing compositions thereby, as described in U.S. Pat. No. 3,968,276. Alternatively, as disclosed in U.S. Pat. No. 3,558,596, reactive flameproofing compositions which are fixed to the wood by reaction can be introduced into the wood.
However, even if no direct water contact is to be feared, for example in the interior area of buildings, it is to be expected with the use of conventional flameproofing compositions that the equilibrium moisture content will be increased. Poorer dimensional stability of the wood components, corrosion at nails, screws and fittings and a higher susceptibility to microbial attack may result therefrom. Thus, it is known from Fire and Materials 2001, 25, 95-104 that owing to their nutrient effect, flameproofing compositions based on ammonium phosphate or urea can promote an attack by wood-decomposing moulds.
Furthermore, a flame-retardant treatment of the wood may have disadvantages in the stability, glue adhesion and coatability of the wood. For example, reactive adhesive or coating systems based on polyurethanes may not react or may do so only to an insufficient extent if the pH of the substrate was changed by the flameproofing composition. The mechanical stability may also deteriorate as a result of a flame-retardant treatment.
In order to meet high fire protection requirements while using a small amount, halogen-containing flameproofing compositions for wood were proposed, for example halogen phosphonates in U.S. Pat. No. 2,725,311 and halogen phosphites and halogen phosphates in U.S. Pat. No. 3,285,774. The halogen content is responsible for good efficiency. The high fume densities and fume toxicities in the event of a fire are disadvantages of these substances. In addition, halogen-containing flameproofing compositions no longer meet society's requirements regarding product safety and environmental protection.
Flameproofing compositions based on ammonium salts of phosphoric acid or polyphosphoric acids are frequently used for the flame-retardant treatment of wood. They have the disadvantage that their flameproofing effect is low and they therefore have to be combined with further substances, for example with boron compounds, in order to improve the effect.
SP REPORT 2006:30 (Swedish National Testing and Research Institute) describes the use of phosphonic acids as flameproofing compositions without giving experimental details. In some experiments in this context, however, serious deficiencies of these flameproofing compositions were evident (cf. Comparative Example V 2)
Stringent and increasingly normative requirements, the change in acceptance of certain chemicals by the public and the described disadvantages of the known flameproofing treatments for wood-base materials lead to a need for novel solutions for the production of flame-retardant wood-base materials. The object of the present invention was to provide compositions and processes for the production of flame-retardant wood-base materials which, on the basis of substances readily available on an industrial scale and in a one-stage treatment, show high efficiency. The compositions sought should be free of halogen-containing substances, free of inorganic phosphates and further co-flameproofing compositions or auxiliaries and should be nonvolatile and readily water-soluble. The properties of the wood, in particular its mechanical stability, should not be adversely affected.